After having anal sex and having feces repeatedly rammed into the penal hole, the penis fills up with a string of feces. Then upon taking a leak for the 1st time after anal sex, the skinny strip of feces will come out into the toilet. That strip is roughly the length and diameter of a chocolate french fry, hence the name.

"I forced Bryce to eat my chocolate french-fry as his punishment for not purging his man-gina before having his little hissy-fit and demanding I put the ol' crank shaft to him."